Generally speaking, Raschel-type warp looms with double or single needlebed are multibar, i.e. they comprise a plurality of thread-guide bars, from a minimum of two to eight and more, placed one beside the other so as to be basically parallel to one another.
As is known, each bar is associated to a plurality of thread-guides whose function is to guide threads during the knitting process. As a rule, each bar is associated to a number of thread-guides corresponding to the number of needles of a single needlebed. For instance, in a knitting machine with a needlebed of about 3.5 meters and a fineness of 24 (i.e. 24 needles pro inch of length), each bar can carry 3,312 thread-guides.
In order to perform their function, the thread-guides have to be threaded with the threads taking part in the knitting process. The threading of each thread-guide depends on the kind of fabric to be obtained. It should be pointed out that not all thread-guides are threaded simultaneously; generally, half of them are threaded, i.e. one or two thread-guide bars of the machine have full threading and the other variable threading.
The threading of the thread-guides is carried out only by hand by the personnel working in the knitwear factory, depending on the technical specifications required by the knitted item.
Threading is an extremely critical and difficult operation due to the large number of thread-guides to be threaded and since the useful space between two adjacent thread-guides is very narrow (less than one millimeter in case of fineness 24). For instance, fully threading a machine equipped with eight thread-guide bars can require a time of about 3 hours for 4 operators working simultaneously.
As is known, in order to facilitate threading combs are used having a plurality of teeth embedded into a lead block and ending up into a hook so as to thread simultaneously more thread-guides, as a rule ten to twenty. Each tooth is led through the eyelet of the thread-guide to be threaded, so as to intercept by means of the hook a thread and introduce it into the eyelet of the thread-guide.
The use of such threading combs enables to make the thread-guide threading process faster but requires that only one clean thread-guide bar gets before the operator. As a consequence, in order to carry out this operation, as a rule the outer bars are dismounted temporarily and threading starts from the inner ones.
This involves an increase of time and costs for dismounting the bars, subsequently re-mounting them and, if necessary, re-calibrating the machine. Moreover, there are still significant problems in case of thread breaks during the knitting step, since the thread has to be threaded again with the bars mounted, thus in a small operating space and trying not to damage the fabric. These drawbacks have been partially solved by known devices, such as the one described in document U.S. Pat. No. 6,289,703, involving the vertical lifting of the thread-guide bars, of the support thereof and of the oscillating shaft, around which said support moves with an oscillating motion for carrying out the “swing” movement and bringing alternatively the threads before and behind the needle hook. The accessibility of the thread-guides is thus improved, especially of those associated to the inner bars, and therefore threading does not require operations involving the temporary dismounting of the outer bars.
However, also these solutions are not without drawbacks. First of all, the structure of the machine becomes very solid and bulky due to the large size of the mechanisms designed to lift the devices dedicated to the “swing” movement since the masses to be lifted are quite high. The lifting of the mechanisms performing the “swing” movement further involves modifications of the anchoring positions of said mechanisms and, therefore, there are inevitably clearances affecting the accuracy of the machine and increasing vibrations, which are already quite difficult to control in case of supports and oscillating shafts firmly anchored to the machine.
As a consequence, the machine becomes more expensive, complicated and difficult to manage and handle.
Moreover, the lifting of the mechanisms dedicated to the “swing” movement is quite small, generally around 10-20 mm, i.e. enough to give a minimum access to the thread-guides of the innermost bars. As a consequence, there is still the problem of the full accessibility and visibility of the inner thread-guides, which, although being lifted with respect to the needles, are parallel to the latter and in order to carry out threading the operators have to work in an uncomfortable and little safe position, since they have to physically lower their heads under the plane of the needles so as to be able to see.